A conventional container used for the vacuum-precooling method is constructed, as shown in FIG. 7, of a container body A made of a foamed synthetic resin and a cover B made of a foamed synthetic resin to be fitted gas-tight on the container body A. There is ventilation through hole C, having a diameter of about 10 mm, in proper positions in the container, in the cover B, for example. Thus, fruits and vegetables are put into the container body A of the refrigerating container. The container is closed with the cover and is placed in a vacuum chamber. When the vacuum chamber lowers its inside pressure to about 3 mmHg, air in the container is forcibly evacuated by ventilation through holes C. Thus, moisture contained in the cooled materials is partially evaporated to derive latent heat for gasification, thereby precooling the materials in the container.
The present Applicant has already disclosed in Japanese Utility Model Publication No. 63-616 this type of container usable for the vacuum precooling method, which has drastically improved the existing technology by forming openings with orifice effects in the vicinity of the fitting portions of the body and the cover.
In the former technology out of the two conventional containers as above mentioned, however, from the instant when the inside of the container is returned to the atmospheric pressure level by causing the inside of the vacuum chamber to restore the original pressure, free air flows are allowed between the inside and outside of the container through the ventilation holes. This is because the ventilation holes have a relatively large diameter. The resulting problems: the temperature of precooled materials gradually approaches the ambient temperature deteriorating the precooling effects and the precooled materials are supplied with oxygen, thereby gradually deteriorating their freshness. In order to solve these problems, therefore, the ventilation holes are sealed from the outside with tape or the like after the precooling operation so that the air flow to and from the inside of the container is blocked. Despite this procedure, however, another problem arises, namely, prolonged working time.
In the latter technology, a great deal of attention in view of potential industrial availability is given since operation without sealing the openings of the container drastically reduces the amount of work. Despite of this advantage, however, the structure may be likely to require "trial-and-error" work to determine orifice shapes. And there is other problem that the orifices acting as the ventilating communication conduits cannot be made longer.